I have access to my university's telescope, Dearborn Observatory, an 18.5 inch refractor on the shore of Lake Michigan, just north of Chicago (yes, it's an atrocious location, but the telescope still works fantastically given this), and I was wondering if it would be possible to detect exoplanet transits. I have access to a CCD. Obviously, normally this would take quite a while, but I was looking at Wikipedia's list of transiting exoplanets and several of them have periods of less than 10 hours or so.

Are there any good resources about the math for this? Or is the approximation that size of the change in light observed by the transit is just using the ratio of the area that the planet occludes to the area of the host star a reasonably good approximation?

Or is observing the transits of any known exoplanets completely unfeasible with this size of telescope? From previous nights of photographing the sky, it seems like the telescope's limit is roughly magnitude 13-15, beyond which the signal-to-noise ratio is just too low to get meaningful data.

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    $\begingroup$ @aventurin I wish I had the reputation to upvote your guys' answers :/ I don't want to accept any quite yet, in order not to deter someone else who may have something else useful to contribute $\endgroup$ Commented Jun 6, 2016 at 1:16
  • $\begingroup$ ^^^ and @James Kilfiger $\endgroup$ Commented Jun 6, 2016 at 1:18
  • $\begingroup$ You don't say what sort of camera and other equipment you have. A CCD camera is required. If you have one, then it is easily feasible. $\endgroup$
    – ProfRob
    Commented Jun 6, 2016 at 4:45
  • $\begingroup$ @RobJeffries Crap, yes I have access to a CCD, I'll edit the questions, thanks! $\endgroup$ Commented Jun 6, 2016 at 5:56
  • $\begingroup$ @Ben Sandeen You're right. I didn't want to deter you. Sorry for that. Deleted my answer and did the upvote for you. $\endgroup$
    – aventurin
    Commented Jun 6, 2016 at 15:28

2 Answers 2


A bright, nearby star with a large exoplanet would be best. For example, the first star to have a transiting planet observed was HD 209458, in Pegasus. It has a magnitude of 7.65. When the large "hot jupiter" transits the star it dims by a relative flux of 0.984. That corresponds to a change in magnitude of about 0.016. I.e. it changes from a magnitude 7.65 star to a 7.67 star.

If you know the radii you can calculate the relative flux during a transit by assuming the planet blocks all the light.

The radius of the planet HD209458b is about 100000km and the star 800000km, so the planet has an area 1/64 of the area of the star's disc, and as expected 1-1/64 = 0.984: which is in good agreement with the light curve.

That won't be noticeable with the naked eye, and is challenging for an amateur. A couple of links:


An 18.5 inch telescope is easily large enough to collect the photons you require. So long as you have a good CCD camera, with a pixel size on the sky that is considerably smaller than the "seeing" and you take a great deal of care in how you make and analyse the observations. Exposure times can be minutes if necessary, because a typical transit lasts a couple of hours.

What I think is necessary though, is to pick targets where there is another star of similar (or maybe you can get away with 1-2 mag fainter) brightness in the same CCD field of view as the target. This can be used (so long as it isn't a variable on hour timescales) to compensate for changes in atmospheric conditions and extinction.

  • $\begingroup$ I wish I could also accept this answer :/ Thank you for the great info! $\endgroup$ Commented Jun 6, 2016 at 16:03

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